3_CH6_diffusion (7) - CH 6: Interactions between Cells and...

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CH 6: Interactions between Cells and the Extracellular Environment I. Extracellular Environment II. Diffusion and Osmosis III. Carrier-Mediated Transport IV. The Membrane Potential V. Cell Signaling (not this)
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ICF 67% ECF 33% I. Extracellular Environment
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Fig 6.1
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Categories of Transport Across the Plasma Membrane Plasma Membrane: selectively permeable.  It is  permeable  to water, O2, CO2, N2, ethanol, urea, and lipid- soluble molecules that cross by simple diffusion. The membrane is  impermeable  to sugars (glucose, fructose),  Carrier-Mediated Transport.  This requires the action of specific carrier membrane  proteins.  e.g., facilitated diffusion (glucose) active transport
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Categories of Transport Across the Membrane: Another Classification Scheme Passive Transport Movement down a concentration gradient; does not require metabolic energy (ATP): e.g., diffusion; osmosis; facilitated diffusion. Active Transport Movement against a concentration gradient; requires  metabolic energy (ATP). Requires specific carrier proteins
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Diffusion Molecules in a gas or dissolved in a solution are in a  constant state of random motion that scatters molecules  evenly. When different concentrations exist then  movement is by  diffusion   down  the concentration  gradient.   Net diffusion – from higher concentration to lower  concentration. Diffusion of water through a membrane = osmosis.
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Fig 6.2
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Fig 6.3: diffusion of oxygen and carbon dioxide
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Hydrophobic lipid-soluble (non-polar) molecules (e.g., steroids) diffuse directly through lipid bilayer. Small uncharged polar molecules (e.g., O 2 , N 2 , CO 2,  urea, ethanol,  glycerol)  can also cross the membrane.  Although  water  is polar it  is small enough to move freely through the lipid bilayer. Hydrophilic (Polar) Molecules Need Help Charged ions: H +  , Na +  , HCO 3  , K +  , Ca 2+  , Cl , Mg 2+ and large hydrophilic (polar) uncharged molecules (e.g., sugars) cannot  diffuse through lipid bilayer and require ion channels or specific  transporters to cross. Diffusion Across The Plasma Membrane  
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Fig 6.4: Ions pass through membrane channels H +  , Na +  , HCO 3  , K +,  Ca 2+  , Cl , Mg 2+ : require special ion channels
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Rate of Diffusion Rate of diffusion depends on: Magnitude of concentration gradient Permeability of the membrane Temperature Surface area of the membrane through which the  substances are diffusing Distance Diffusion time increases in proportion to the square  of the distance over which the molecule must diffuse.
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Osmosis The net diffusion of  water (the solvent ) across a selectively  permeable membrane. Some cells have special water channels  ( aquaporins ) that allow water to move through freely. Two requirements of osmosis:
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This note was uploaded on 02/12/2012 for the course BI 202 taught by Professor Winn during the Fall '10 term at Northern Michigan University.

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3_CH6_diffusion (7) - CH 6: Interactions between Cells and...

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